تجزیه و تحلیل مدلسازی و عملکرد یک واحد احیا کننده آب شیرین کن خورشیدی

In this paper, a regenerative solar desalination unit is modeled and its performance evaluated. The unit consists of two basins (effects), with provision for cooling water to flow in and out of the second effect. This arrangement has the advantages of increasing the temperature difference between water and glass cover in the first effect and utilizes the latent heat of water vapor condensing on the glass of the first effect to produce more fresh water in the second effect. The performance of the regenerative still is evaluated by comparison with the performance of the conventional still under the same weather conditions. The results of the simulations show that the productivity of the regenerative still is 20% higher compared to the conventional still. Making the stills perfectly insulated increases their productivity two and one half folds. Insulation has higher effect on the regenerative still compared to the conventional still. The wind speed has a significant effect on the productivity of the stills; it can increase the productivity by more than 50% if the wind speed is increased from 0 to 10 m/s. The thickness of water on top of the first glass cover and the mass flow rate of water going into the second effect have marginal effect on the productivity of the regenerative still.

Solar distillation is one of the thermal desalination methods that attracted researchers’ attention due to its potential application in remote locations far from electricity grid. The simple solar still (henceforth conventional) consisting of a water basin and a single glass cover is the first proposed design of solar still that is easy to construct and has virtually no operating cost. However, even in areas of relatively high levels of solar insolation its annual performance per square meter of aperture is limited to an average of about 3 l/day. This motivated the search for methods to enhance the still productivity. These methods fall into two separate categories: active and passive. Among the active methods used is cooling of the glass cover and/or utilizing the latent heat of condensation dissipated through the glass cover. The glass cover gains heat from water vapor condensation on the lower side and loses this heat to the ambient by convection and radiation. Normally the rates of heat transfer from the glass cover to the ambient by convection and radiation are small, especially at low wind speeds. This causes the glass temperature to remain high which adversely affects the still productivity. Cooling of the glass cover increases the temperature difference between water in the basin and the glass cover, which increases the rate of evaporation, giving higher still productivity. Several theoretical and experimental studies were conducted on cooling the glass cover by flowing water film on the cover [1], [2], [3], [4], [5] and [6]. All those studies indicated increase in the productivity, some reported up to 20%. Another technique used to cool the glass cover is by passing water between double-glass cover. There are two different arrangements for the double glass cover [6]. One arrangement leaves no space between the flowing water surface and the upper glass cover, thus no evaporation from the enclosed water takes place. This arrangement is designed to cool the glass cover. Depending on the flow rate and the exiting water temperature, the heated water in the double glass channel can be fed into the basin for heat recovery and/or to remove the brine from the basin to avoid scale accumulation. This double glass channel arrangement was investigated by Abu-Arabi et al. [7] who found significant improvement over the conventional still. Singh and Tiwari [8] studied the thermal performance of a similar still working under thermosyphon mode of operation and found improvement in productivity. They also connected the still to a heat exchanger–collector arrangement to increase the basin water temperature. In the second arrangement the spacing can also be made in such a way to have a second basin (or second effect) with and without flowing water. In this case evaporation takes place from the water surface on top of the lower glass cover and additional distillate can be collected from condensing the water vapor on the upper glass and at the same time the exiting heated water can be fed to the lower basin. According to Tiwari [6], Parkash and Kavanthekar [9] investigated this arrangement. However in their mathematical model, they neglected the heat capacity of glass covers and flowing water in addition to the absorption of solar radiation in the flowing water. Furthermore, no detailed results on the performance of such still were reported. The focus of this study is on the regenerative solar still, which does both; cooling the glass cover and utilizes the latent heat of condensation to produce more desalinated water in a second effect. The regenerative solar still is mathematically modeled by writing energy balance equation for each of the system components, namely; basin, water in basin, first glass cover, water film on top of first glass cover, and second glass cover. The heat capacity of glass covers and water film is included, which has been neglected in previous studies. Effect of key parameters on the productivity of the still is also investigated under the climatic conditions encountered in Muscat, Oman.

The regenerative solar still is modeled using unsteady state analysis and the resulting system of nonlinear ordinary differential equations solved using finite-difference method in implicit scheme coupled with tridiagonal matrix algorithm. The parameters affecting the performance of the still are identified and performance assessments were carried out by comparing with the results for the conventional solar still. It is concluded that the productivity of the regenerative still is more than 20% higher than that for the conventional still. Making the stills perfectly insulated increases their productivity two and one half folds. There is more positive effect due to insulation on the regenerative still compared with the conventional still. The wind speed has a significant effect on the productivity of the stills, it can increase the productivity by more than 50% if the wind speed increased from 0 to 10 m/s. The thickness of water on top of the first glass cover and the mass flow rate of water going into the second effect have marginal effect on the productivity of the regenerative still.